Pay-as-you-go cell phone provider Straight Talk employs SIM cards, allowing you to use your own unlocked GSM or CDMA phone on the network or purchase one of the phones that the company offers directly. However, not all GSM phones will work with Straight Talk's service, so purchasing a SIM card directly from the company may not work with your existing phone.

SIM Card Compatibility

In general, if you have a phone that already uses a SIM card, it should work on Straight Talk. However, international phones may not be compatible if they do not work on the 850 or 1900 MHz frequencies. CDMA phones that run on 4G LTE networks are also compatible because they use SIM cards. These phones require an additional registration step to work on the network. Furthermore, 3G CDMA phones from networks such as Verizon and Sprint are not compatible with Straight Talk.

Unlocking Phones

If you have a GSM phone from a provider such as AT&T or T-Mobile, you must first unlock it before you can take your phone to the Straight Talk network. You will have to contact your provider to have them unlock the phone, which allows you to take it to the new network. Your provider may charge a fee or require that you've been with the company for a certain amount of time before you can unlock your phone.

Micro-SIM Compatibilitty

You may have an unlocked GSM phone that uses a micro-SIM card. This is the same technology that allows phones to connect to cell networks in a smaller form factor. Straight Talk and Wal-mart periodically sell these cards on their websites. However, if a micro-SIM is not available, you can purchase a SIM card cutter and cut the original-sized SIM card to use in your phone's micro slot. Beware that this may void your warranty on the card, however.

Considerations

Whether you purchase a Straight Talk SIM card or a CDMA access code, you will pay a one-time fee per phone to switch your phone to the network. At time of publication, this fee is $15. This is in addition to the monthly fee you must pay to use your phone with Straight Talk. Straight Talk's SIM cards are not compatible with the $30 All Your Need Plan. For this plan, you must purchase a phone from the provider's website or Wal-mart, which exclusively sells Straight Talk phones. If you purchase your SIM card for Straight Talk from Wal-mart, you can choose between AT&T and T-Mobile's networks, which may be beneficial if you live in an area with little or no coverage from either provider.

About the Author

Nicole Martinez began writing in 2010 and has since been published on various websites. She primarily writes about computer- and internet-related topics, especially those concerning website maintenance and programming.